A display device, such as a liquid crystal display device or an organic EL display device, uses a pressure-sensitive adhesive to bond one of various types of transparent optical elements, such as a polarizing film, a retardation film, and a transparent cover member including a cover glass, to another optical element. More specifically, a pressure-sensitive adhesive layer is disposed between two optical elements to be bonded together, and then the two optical elements are pressed against each other, so that they are bonded together through the pressure-sensitive adhesive layer to thereby form a laminate of optical elements. The laminate of the optical elements configured as above may be installed such that the transparent optical element is located on a viewing side of the display device with respect to the pressure-sensitive adhesive layer and the other optical element. Such type of display device has a problem in that, when outside light enters from the transparent optical element located on the viewing side, the incident light is reflected at an interface between the pressure-sensitive adhesive layer and the other optical element located on a back side, and returned to the viewing side. This problem becomes prominent when an incident angle of outside light is relatively large.
On the other hand, in a touch panel-equipped display device which has been increasingly becoming popular in these years, a transparent electrically conductive layer, such as a patterned ITO (Indium Tin Oxide) layer, is formed on a surface of an optical element to which the first mentioned transparent optical element is bonded. It has been recognized that this type of display device further has a problem of “pattern visibility” in which the pattern of the transparent conductive layer becomes visible from the viewing side under the influence of internal reflection of the incident light at the interface between the pressure-sensitive adhesive layer and the transparent conductive layer.
In each of the cases, the internal reflection is caused by a difference in refractive index between the pressure-sensitive adhesive layer and either one of the optical elements which are bonded together by the adhesive. JP 4640740 B (Patent Document 1) teaches a technique for addressing this problem. More specifically, the Patent Document 1 discloses a pressure-sensitive adhesive composition capable of reducing total reflection of light at an interface between the transparent optical element and the pressure-sensitive adhesive layer and at the interface between the pressure-sensitive adhesive layer and the other optical element. The composition disclosed in the Patent Document 1 is allegedly described as having a refractive index which is high in its dried and/or cured state, such that the refractive index is close to refractive index of the transparent optical element and that of the other optical element. The teaching of the Patent Document 1 is that a pressure-sensitive adhesive layer for bonding two optical elements together is entirely formed to have a refractive index close to refractive indexes of the two optical elements.
The teachings of the Patent Document 1 may be effective in suppressing the interface reflection, however, there is a problem in that the composition itself becomes costly, because the teachings are based on the use of a particular monomer component.
JP 5564748 B (Patent Document 2) discloses a pressure-sensitive adhesive having a pre-adjusted refractive index and comprising particles of zirconium oxide or titanium oxide having an average dispersed particle size of 1 nm to 20 nm, the particles of zirconium oxide or titanium oxide being dispersed in a layer of a transparent pressure-sensitive adhesive throughout the thickness of the layer. It is believed that this pressure-sensitive adhesive can suppress the aforementioned interface refraction, since particles of zirconium oxide or titanium oxide having a high refractive index are mixed with the transparent pressure-sensitive adhesive material such that the overall refractive index of the layer of the adhesive is thus increased. However, the teachings of the Patent Document 2 require a use of a large amount of high refractive index material. This may cause deterioration in properties required as a pressure-sensitive adhesive, and an increase in cost. Moreover, the high refractive index material recommended in the Patent Document 2 is an inorganic material in a form of particles. Thus, another problem may be encountered in that the particles are hardly dispersed, so that whitish haze may be produced due to light scattering. In this regard, one may consider using particles of an organic material. In this case, however, it becomes difficult to solve a problem of coloration.
With a view to improving the technique disclosed in the Patent Document 2, JP 5520752 B (Patent Document 3) proposes to have metal oxide particles which are to be dispersed in a pressure-sensitive adhesive, coated with a polymer. Teaching of the Patent Document 3 is that, although the pressure-sensitive adhesive layer in the Patent Document 2 has a problem of deterioration in adherence property thereof due to the metal oxide particles exposed to a surface of the pressure-sensitive adhesive layer, this problem can be solved by coating the metal oxide particles with a polymer. The technique proposed by the Patent Document 3 would have a potential to improve the adherence property of the pressure-sensitive adhesive layer to some extent. However, it cannot solve most of the remaining problems pointed out in connection with the Patent Document 2. In particular, the technique proposed by the Patent Document 3 comprises coating the metal oxide particles with a specific polymer. Thus, the resulting pressure-sensitive adhesive becomes more costly than that in the Patent Document 2.